1. Field of the Invention
This invention relates to a perpendicular magnetic recording medium, mounted in an external storage device for a computer or in other various magnetic recording devices.
2. Description of the Related Art
Perpendicular magnetic recording methods are beginning to be commercialized as a technology to achieve high densities in magnetic recording, in place of conventional longitudinal magnetic recording methods.
A perpendicular magnetic recording medium is generally formed by sequentially layering a soft magnetic backing layer, a nonmagnetic intermediate layer, a magnetic recording layer, and a protective layer. As the nonmagnetic intermediate layer, Ru or an alloy containing Ru is used. As the magnetic recording layer, materials having a so-called granular structure, which comprise ferromagnetic crystal grains mainly comprising CoPt alloy having ferromagnetic properties, and nonmagnetic grain boundaries mainly comprising an oxide and surrounding the ferromagnetic crystal grains, are widely used.
For example, a perpendicular recording medium, having a nonmagnetic intermediate layer of Ru and a magnetic recording layer of CoPtCr—O alloy with a so-called granular structure, is described in “High Performance CoPtCrO Single Layered Perpendicular Media with No Recording Demagnetization”, Oikawa et al, IEEE Transactions on Magnetics, Vol. 36, No. 5, p. 2393-2395, September 2000. Here, as the film thickness of the Ru which is the nonmagnetic intermediate layer is increased, the c-axis orientation of the CoPtCr—O alloy in the magnetic recording layer is improved, isolation of magnetic crystal grains is promoted, and as a result, excellent magnetic characteristics and electromagnetic transducing characteristics are obtained.
Further, in U.S. Pat. No. 7,067,206, a perpendicular magnetic recording medium is disclosed having, in order, a soft magnetic backing layer, an orientation control layer comprising an alloy with a face-centered cubic (fcc) structure, a nonmagnetic intermediate layer of Ru having a hexagonal close-packed (hcp) structure, and a magnetic recording layer of a CoPtCr—SiO2 alloy having a granular structure. Here, by providing an orientation control layer, further control of the fine structure in the nonmagnetic intermediate layer and magnetic recording layer is possible, so that even more satisfactory electromagnetic transducing characteristics can be realized.
In Japanese Patent Application Laid-open No. 2004-288348, a perpendicular magnetic recording medium is disclosed in which an underlayer comprising a soft magnetic Permalloy material and a soft magnetic Co layer or a soft magnetic Co-base alloy layer are provided between a soft magnetic backing layer and a nonmagnetic intermediate layer comprising Ru or Ru-base alloy. Here, by providing a soft magnetic Co layer or a soft magnetic Co-base alloy layer, the coercivity of the magnetic recording layer is increased, the squareness ratio of the medium is improved, and simultaneously the thickness of the Ru or Ru alloy film serving as the nonmagnetic intermediate layer can be reduced. As a result, media noise is decreased and the SNR is improved.
On the other hand, in Japanese Patent No. 3588039, a perpendicular magnetic recording medium is disclosed in which a nonmagnetic intermediate layer comprising CoCr alloy having a hcp structure is provided between an underlayer film and the perpendicular magnetic film.
Further increases in recording density are sought for magnetic recording media, and whereas the recording density of the first perpendicular magnetic recording media commercialized using the above-described technology was about 140 Gbits/in2, at present recording densities several times this or more are being demanded. To this end, further improvement of electromagnetic transducing characteristics, that is, improvement of the signal-to-noise ratio SNR upon recording and reproduction using magnetic heads, is sought.
In order to improve the SNR, promotion of the magnetic isolation of magnetic crystal grains in the magnetic recording layer (promotion of separation of oxide grain boundaries and alloy crystal grains), finer crystal grain diameters, greater homogeneity, and improvement of c-axis orientation are deemed necessary, and to this end, precise structural control of the nonmagnetic intermediate layer is necessary. In the technology of the prior art disclosed in U.S. Pat. No. 7,067,206 above, an orientation control layer is used for the structural control of the nonmagnetic intermediate layer; but it is becoming difficult to realize higher recording densities by this means alone. And, in the technology of the prior art disclosed in Japanese Patent Application Laid-open No. 2004-288348 above, a Co layer having soft magnetic properties or a Co-base alloy layer having soft magnetic properties is provided between the nonmagnetic intermediate layer and the soft magnetic backing layer, but magnetic recording media in which a soft magnetic Co layer is provided has the problem of poor corrosion resistance (large amounts of Co elution).